The University of Texas Southwestern Medical Center in Dallas has initiated a major expansion in its NMR faculty and facilities. To drive this program, the University has committed over $5M for purchase of additional instruments, construction of state-of-the-art facilities, hiring of dedicated support personnel and appointment of senior faculty. Integral to this program is the purchase of an 800 MHz spectrometer equipped with a cryoprobe to support new research initiatives that cannot be carried out on lower field instruments, and funds are requested to assist in this endeavor. This instrument will support the research of 30 users in the four principal NMR groups in the Departments of Biochemistry and Pharmacology, plus numerous collaborators and minor users within and outside the university. Research using the instrument will address central questions in a broad range of biological areas, including molecular mechanisms of neurotransmitter release and constitutive intracellular membrane traffic, control of actin cytoskeletal architecture and dynamics through integrated signaling networks, regulation of cellular hypoxia responses and circadian cycling, and molecular mechanisms of a mitotic checkpoint that ensures the high-fidelity segregation of chromosomes in mitosis. Specific structural/biophysical problems in these areas include studies of the calcium sensor, synaptotagmin, and its interaction with SNARE proteins, essential components of the membrane fusion machinery; the activation mechanism of the 240 kDa Arp2/3 complex, the central actin nucleating machine in the cell, the biophysical foundations of signal integration by autoinhibitory proteins; the ability of PAS domains to control biological function through changes in protein structure and dynamics induced by natural and non-natural ligands; competition for the mitotic checkpoint protein Mad2 by its upstream regulator, Mad1, and its downstream target, Cdc20. These studies will require NMR analyses of multi-component systems ranging in size from 15-250 kDa, and will involve innovative use of CRIPT/TROSY methods, lanthanide-assisted magnetic field alignment, multi-field dynamics studies and analysis of very dilute solutions, necessitating maximum field strength and sensitivity. In summary, an 800 MHz spectrometer at UT Southwestern will foster cutting edge science in important biological areas that will expand the boundaries of NMR methods to large and/or complex systems. The instrument will be strongly supported by the University, and administered through a detailed plan that ensures efficient use and maintenance, specific mechanisms for training new users and dissemination of results to the larger communitv.